Coastal Landscape Evolution - Tsunami - page 62

landward flow under a storm wave rarely exceeds 7-8 s.
This is not sufficient to generate bedrock sculpturing.
Landward flow in the tsunami window lasts for tens of
minutes. This gives enough time for high-velocity flow to
sculpture highly resistant bedrock into the features descri-
bed in the previous chapter. Tsunami appear to be the only
mechanism capable of providing these conditions along the
coast.
The effect of off-surge (backwash) in tsunami is rarely
mentioned in the literature. Near the coast, such offshore
flow is generally channelised as the volume of overwash
drains seaward through inlets and along defined drainage
channels. More important, undertow whereby flow in the
water column moves seaward along the seabed can occur
out to the shelf edge in depths of 100-130 m of water.
Continental shelf profiles may be a product of repetitive
combing by tsunami-induced currents (Coleman 1968 ). If
this undertow contains any sediment, then it behaves as a
density current and powerful ebb currents of 2-3 m s -1 can
sweep down the continental slope and along the abyssal
plain under the passage of a tsunami wave. These currents
are two-to-three times greater than those observed for
storms (Morton 1988 ). They can scour the seabed and
deposit sand and gravels considerable distances from the
shelf edge. Such flows may account for the presence of
coarse, winnowed, lag deposits and 2-3 m diameter boul-
ders at the toe of the continental slope in water depths of
200-400 m (Shiki and Yamazaki 1996 ).

differences from those deposited by storm waves (Morton
et al. 2007 ). First, sandy tsunami deposits rarely exceed
25 cm thickness, while storm deposits can be up to 10 times
thicker than this. Second, tsunami can deposit layers of
sediment up to several kilometers inland from the coast,
whereas storms appear to build up an asymmetric wedge of
coarse-grained sediment or berm that rarely extends
50-100 m inland (Dawson 1999 ). Third, tsunami deposits
drape over the landscape, while storm deposits tend to fill in
the depressions. Fourth, while both storm and tsunami
deposits contain sand originating from beaches or dunes,
tsunami deposits can contain finer sediment brought from
the continental shelf. The presence of angular rip-up clasts
of mud or mud layers is characteristic of tsunami deposits
(Kortekaas and Dawson 2007 ). Mud is winnowed from
storm deposits by repetitive backwash while mud clasts are
rounded within short distances by repeated contact with the
bed. Fifth, a single-bed tsunami deposit is massive with
little evidence of bedding while a storm deposit tends to be
bedded. Layers of fining upward sediment may be present in
sandy tsunami deposits; but such layers may not be visually
evident or related to the number of tsunami waves. In storm
overwash deposits, such layers relate to individual waves
and appear as beds. Where tsunami deposit multiple beds,
layers show repeated alternation of current directions cap-
ped by mud drapes (Fujiwara 2008 ). The layers fine and thin
upwards reflecting the changes of wave amplitude with
time. The mud drapes are formed by suspension fallout
between successive tsunami waves. Sixth, tsunami deposits
along rocky coasts can contain a significant fraction of
broken, angular, and rounded pebbles (Bryant et al. 1996 ).
Breakage is produced by intense turbulence as the tsunami
makes contact with rocky shores or rips up material from
shore platforms. Storm deposits may contain some angular
and broken pebbles, but the amounts are much less. Finally,
tsunami deposits can contain unbroken shell and forami-
nifera because flow may be laminar and dominated by
suspension. This hinders particle-to-particle contact or
particle contact with the bed. Storm waves carry particles in
traction. Contact between particles and the bed are frequent
such that shell and foraminifera show signs of fracturing and
abrasion.
Gravel and cobble storm-built beaches tend to be char-
acteristic of eroding coasts, although there are exceptions.
Thus, their preservation potential is poor. Coarse-grained
tsunami beach deposits have a higher potential for preser-
vation, if not in the longer geological record, then certainly
at high sea level stillstands over the last few million years
on tectonically stable coasts. These sediments often are
deposited above the limits of storms, and unless eroded by
subsequently larger tsunami, will remain stranded above the
active coastal zone on such coasts.

4.3.1 The Nature of Tsunami Versus Storm

Deposits

Storms are mainly responsible for two types of deposits:
beaches consisting of gravels, cobbles, and boulders
(Bourgeois and Leithold 1984 ), and overwash sand splays
(Morton et al. 2007 ). Unless storm waves can overwash a
beach, sand is generally moved shoreward only by fair-
weather swell (Komar 1998 ). Gravel and cobble beaches are
characterized by shape and size sorting of particles (Bour-
geois and Leithold 1984 ). Larger disc-shaped particles tend
to move to the top of the beach where they are deposited as
a berm that may develop at the limit of storm-wave run-up.
Smaller spherical particles tend to accumulate at the base of
the foreshore. This difference is due to the greater potential
for suspension transport of discs in swash and the greater
rollability of spheres back down the beach face under
backwash. Where sand is available, it tends to be trapped
between the larger particles or accumulate at the bottom of
the beach.
While tsunami can also overwash coasts and transport
coarse sediment, the internal fabric of the deposits shows